Project Summary During tissue repair, many stem cell populations undergo a dynamic phenotypic change from a quiescent state to an activated state. In muscle stem cells (MuSCs), this dynamic activation process is essential for effective tissue regeneration. Despite the conserved nature of these activation processes, the dynamics of stem cell activation and their contribution to disease states remains largely unknown. We have generated single cell assays that allowed us to study state transitions of adult and aged MuSCs during activation. These results support a conceptual view of the aged stem cell phenotype as a combination of pathological steady-states and deficiencies in cell state dynamics. This provides us with the opportunity to identify factors that rejuvenate MuSC function during aging. In this project we will examine the role of physiological rejuvenation interventions on MuSC heterogeneity and activation state transitions. Understanding how rejuvenation interventions control MuSC activation response is critical for the effective treatment of the ever-expanding aged population.